s390/kasan: remove identity mapping support

The identity mapping is created in the decompressor,
there is no need to have the same functionality in
the kasan setup code. Thus, remove it.

Remove the 4KB pages check for first 1MB since there
is no need to take care of the lowcore pages.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>